The present disclosure relates to an image reader that reads a document sheet by scanning and is used in apparatuses such as, for example, digital copiers and image scanners, and an image forming apparatus including the same.
Some conventional image readers incorporated in multifunction peripherals and the like employing an electro-photographic process are provided with an automatic document feeder configured to convey document sheets one by one to a document table (a contact glass) for reading, the document sheets each being discharged from the document table after being read. In such an image reader, two reading methods can be performed, namely, a sheet through method and a sheet stationary method. In the sheet through method, document reading is performed by automatically feeding document sheets by means of an automatic document feeder with a reading module maintained stationary at an image reading position. In the document stationary method, document reading is performed by moving the reading module for scanning, and a user opens and closes a document presser upon every completion of reading in order to replace a document sheet on the document table (contact glass) one by one.
In recent years, the mainstream of the sheet through method mentioned just above has been a simultaneous two-side reading method in which images printed on front and back sides of a two-sided document sheet are read simultaneously by using different reading modules in one document conveyance operation performed by an automatic document feeder.
For correction of uneven illumination in a main scanning direction attributable to a light source incorporated in a reading module, lens vignetting, and uneven pixel sensitivity of a reading sensor, before reading a document sheet, shading correction is performed in which white reference is acquired by reading a white reference plate by means of a reading module, while black reference is acquired with a light source of a reading module turned off.
In such an image reader, when continuously reading a plurality of document sheets, image quality may be degraded due to, for example, image density variation caused by a temperature rise in a reading sensor or a light source, or a streaked image caused by adhesion of foreign objects such as paper powder to an image reading position.
There is known a method of reducing image density variation, against temperature rise in a reading sensor or a light source by, for example, correcting white reference data and black reference data in an interval between continuously conveyed document sheets based on a value read by the reading sensor. There is also known a method of reducing occurrence of streaked image, against adhesion of a foreign object such as paper powder to an image reading position by, for example, changing the reading position based on a value read by the reading sensor in an interval between document sheets conveyed in a continuous manner.